Conventional photographic materials based on silver halide are used for a large variety of applications. For instance, in the pre-press sector of graphic arts rather sensitive camera materials are used for obtaining screened images. Scan films are used for producing colour separations from multicolour originals. Phototype setting materials record the information fed to phototype- and image setters. Relative insensitive photographic materials serve as duplicating materials usually in a contact exposure process. Other fields include materials for medical recording, duplicating and hard copy, X-ray materials for non-destructive testing, black-and-white and colour materials for amateur- and professional still photography and materials for cinematographic recording and printing.
Silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view.
In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
A dry imaging system known since quite a while is 3M's dry silver technology. It is a catalytic process which couples the light-capturing capability of silver halide to the image-forming capability of organic silver salts.
Another type of non-conventional materials as alternative for silver halide is constituted by so-called photo mode materials based on photopolymerisation. The use of photopolymerizable compositions for the production of images by information-wise exposure thereof to actinic radiation is known since quite a while. All these methods are based on the principle of introducing a differentiation in properties between the exposed and non-exposed parts of the photopolymerizable composition e.g. a difference in solubility, adhesion, conductivity, refractive index, tackiness, permeability, diffusibility of incorporated substances e.g. dyes etc.. The thus produced differences may be subsequently employed in a dry treatment step to produce a visible image and/or master for printing e.g. a lithographic or electrostatic printing master.
As a further alternative for silver halide chemistry dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. These types of dry imaging elements called heat mode materials (or thermal imaging materials, thermal recording materials or thermographic materials) offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor is any other protection from ambient light needed. Heat mode recording materials, based on change of adhesion, are disclosed in e.g. U.S. Pat. No. 4,123,309, U.S. Pat. No. 4,123,578, U.S. Pat. No. 4,157,412, U.S. Pat. No. 4,547,456 and PCT publ. Nos. WO 88/04237 and WO 93/03928, and international appl. No. PCT EP94/02063. In a preferred embodiment such a thermal imaging medium comprises a transparent support and an imaging layer containing carbon black, optionally additional layers and a stripping sheet. By the conversion of intense laser light into heat on information-wise exposure a surface part of the support liquefies and firmly locks the carbon black, so that after delamination a negative carbon black image is formed on the support.
With several kind of materials which are exposed by specular laser radiation through a transparent support, whether being conventional silver halide or non-conventional materials, the following problem arises. Transparent polymeric resin supports such as polyethylene terephthalate supports tend to contain microscopic dust particles, or catalyst rest particles, or microscopic voids (so-called fish-eyes) which scatter the incoming laser beam so that it does not reach the radiation sensitive layer anymore at the proper location. In negative working systems this leads to the formation of so-called pinholes; in positive working systems it causes the formation of so-called pinpoints. The same phenomenon is caused by the presence of dust or scratches on the surface of the support or in the optionally present subbing layer. This defect is particularly striking in negative working heat mode systems, bases on change of adhesion as described above, where the pinholes become apparent after the delamination step. The defect is most disturbing in recorded full areas, where the pinholes appear as tiny white spots on a black background, and less in recorded separate lines and dots. Although these pinholes, depending on their size are hardly disturbing for practical applications of the finished image, e.g. as a master for the exposure of a printing plate or of a duplicating material, they give the image an unsatisfactory outlook, especially when inspected by means of a magnifying glass.
It is an object of the present invention to provide a method for the formation of an image which is substantially free of the pinhole or pinpoint defect.
It is a further object of the present invention to provide an improved method for the formation of a heat mode image, based on change of adhesion, which is substantially free of pinholes.
Other objects of the invention will become clear from the description hereinafter.